ASSOCIATION AND DISSOCIATION RATE CONSTANTS OF THE COMPLEXES BETWEEN VARIOUS CARDIAC MONOGLYCOSIDES AND Na, K‐ATPase*

Abstract

53 706 binations, the followings have been reported: Mg,3 Mg + ATP,3 Mg + Pi,3v6Mg + Na + ATP,2*3 Mg + a~e t a t e ,~~~ Mg + a r~e n a t e ,~ Mg + Na + nucleotide,69" Mn++,3 Na + Sr,3 Mg + p-nitrophenylphosphate,8>12 and Mg + acetyl phosphate.I2 The two ligand systems that are most effective are the Mg and Pi system and the Na, Mg, and ATP system.

Diverse Toxicity Associated with Cardiac Na+/K+ Pump Inhibition: Evaluation of Electrophysiological Mechanisms

“…The onset of Na ϩ /K ϩ pump inhibition was faster for PST2744 than for digoxin. This is consistent with the previous observation that association and dissociation rate constants of digitalis compounds to the Na ϩ /K ϩ pump are decreased by the glycosidic group (aglycones bind faster than the respective glycoside) (Yoda, 1974), which is not present in PST2744 structure.…”

New Molecular Determinants Controlling the Accessibility of Ouabain to Its Binding Site in Human Na,K-ATPase α Isoforms

“…Results from previous kinetic studies of ouabain binding, which were performed mainly on Na,K-ATPase ␣1 isozymes, have suggested that the rate of association of ouabain to enzymes from different sources, both sensitive or insensitive, is similar and that the ouabain sensitivity is mainly determined by differences in the dissociation rate (Yoda, 1974;Akera and Brody, 1977). Our observation that the human ␣2 isozyme exhibits 5-to 10-fold faster ouabain dissociation and association rates than the ␣1 isoform, despite their similar high ouabain sensitivity, indicates that this prediction is not valid for ␣ isoforms other than ␣1 isoforms.…”

“…It has been postulated that the interaction of cardiac glycosides with the Na,K-ATPase occurs in at least two steps: an initial rapidly reversible binding step, followed by a conformational change that permits the formation of a more stable ouabain-enzyme complex (Yoda, 1974). In this model, differences in both the association and dissociation rates in the absence of a significant difference in the apparent ouabain affinity, as observed for ␣1 and ␣2 isoforms, could be explained by a difference in the flexibility or the accessibility of the ouabain binding site in the two isoforms.…”

“…Experimental and modeling data suggest that cardiac glycosides bind to the extracellular surface of the ␣-subunit (Croyle et al, 1997;Middleton et al, 2000;Farr et al, 2002) and that the physical binding site for cardiac glycosides may be formed by the M3/M4 and M5/M6 hairpins (Koenderink et al, 2000). The association rate of ouabain seems to be dependent on the steroid moiety, whereas the dissociation rate depends both on the steroid and the sugar moieties (Yoda, 1974;Kawamura et al, 2001). For ␣1 isoforms, ouabain association rates are slow and independent of their sensitivity to cardiac glycosides, whereas dissociation rates are low in sensitive isoforms but high in resistant ␣1 isoforms (Yoda, 1974).…”

“…The association rate of ouabain seems to be dependent on the steroid moiety, whereas the dissociation rate depends both on the steroid and the sugar moieties (Yoda, 1974;Kawamura et al, 2001). For ␣1 isoforms, ouabain association rates are slow and independent of their sensitivity to cardiac glycosides, whereas dissociation rates are low in sensitive isoforms but high in resistant ␣1 isoforms (Yoda, 1974). In view of these data, it may be predicted that the differences in the ouabain association and dissociation rates in human ␣1 and ␣2 isoforms, which have a similar high sensitivity to cardiac glycosides, reflect differences in the accessibility of ouabain to its binding site rather than an alteration of the binding site itself.…”